Using this flag will cause the linker to ignore any libraries
that have been listed on the command line but whose symbols are
not actually used.
Right now, it just gets rid of the unnecessary linking against
xcb-render (see https://bugs.debian.org/829709), but it might
catch missing dependencies in the future, and it will generally
keep the resulting binaries nice and tidy.
Ubuntu and Arch are both using --as-needed by default already,
and other distributions will probably switch at some point.